Very long optical fiber transmission paths, such as those employed in undersea or transcontinental terrestrial lightwave transmission systems which employ optical amplifier repeaters, are subject to decreased performance due to a host of impairments that accumulate along the length of the optical fiber composing the transmission path. Typically, in such long optical transmission systems, these impairments vary with time and cause a random fluctuation in the signal-to-noise ratio ("SNR") of the received signal. This random fluctuation contributes to a phenomenon known as signal fading. Signal fading can result in an increased bit error rate ("BER") for digital signals transmitted via the optical fiber path. When the SNR of a digital signal within such a transmission system becomes unacceptably small (resulting in an undesirably high BER), a signal fade is said to have occurred. Experimental evidence has shown that polarization dependent effects, induced by the optical fiber itself and/or other optical components (e.g., repeaters, amplifiers, etc.) along the transmission path, contribute to signal fading and SNR fluctuations. In particular, one of these effects has now been identified as polarization hole-burning ("PHB"), which is related to the population inversion dynamics of the optical amplifiers. A discussion of hole-burning is provided by D. W. Hall, R. A. Haas, W. F. Krupke, and M. J. Weber in "Spectral and Polarization Hole Burning in Neodymium Glass Lasers," IEEE Journal of Quantum Electronics, Vol. QE-19, No. 11, November 1983.
PHB reduces gain of the optical amplifiers within the long haul transmission system for any signal having a state of polarization ("SOP") parallel to that of the primary optical signal carried by the transmission system. However, the gain provided by these amplifiers for optical signals having an SOP orthogonal to that of the primary signal remains relatively unaffected. In simplified terms, the primary optical signal produces an anisotropic saturation of the amplifier that is dependent upon the SOP of the primary optical signal. The anisotropic saturation reduces the population inversion within the amplifier, and results in a lower gain for optical signals having the same SOP as the primary optical signal. This effectively causes the amplifier to preferentially enhance noise having an SOP orthogonal to that of the primary signal. This enhanced noise lowers the SNR of the transmission system and causes an increased BER.
Prior methods for reducing signal fading have included the use of systems that actively adjust the SOP of a signal launched into a given optical path, as a function of the quality of the signal received at the far end of the path. In some methods the SOP of the signal is scrambled. For example, systems are known for scrambling the SOP at frequencies both lower and higher than the bit rate. However, scrambling at frequencies lower than the bit rate causes AM modulation on the data signal within the receiver's bandwidth, thus reducing the potential improvement that can be achieved with low frequency scrambling. Scrambling at frequencies higher than the bit rate can reduce the AM modulation but causes an increase in the transmitted bandwidth, which can also degrade performance.